1. Field of the Invention
The present invention relates to a mobile telecommunication system, and more particularly to a method and an apparatus for conducting communication by using a Hybrid Automatic Retransmission reQuest (HARQ).
2. Description of the Related Art
As generally known in the art, the UMTS (Universal Mobile Telecommunication Service) system refers to the 3rd generation asynchronous mobile telecommunication system, which is based on the European mobile telecommunication systems, particularly the GSM (Global System for Mobile Communications) and the GPRS (General Packet Radio Services), and which employs Wideband Code Division Multiple Access (W-CDMA).
The 3GPP (3rd Generation Partnership Project), which is in charge of the UMTS standardization, is currently discussing Long Term Evolution (LTE) as the next generation mobile telecommunication system of the UMTS system. The LTE, which is expected to be commercialized in about 2010, refers to technology for implementing high-speed packet-based communication having a transmission rate of a maximum of 100 Mbps. Various schemes are being studied to this end. For example, it has been proposed to simplify the network structure so that the number of nodes existing on the communication channels is reduced. A scheme for bringing radio protocols the closest to radio channels is also under discussion.
FIG. 1 shows the structure of an exemplary evolved mobile telecommunication system.
Referring to FIG. 1, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified into a two-node structure including Evolved Node Bs (ENBs or node Bs) 120, 120, 124, 126, and 128 and anchor nodes 130 and 132. The anchor nodes 130 and 132 may be defined as Evolved Gateway GPRS Serving Nodes (EGGSNs). A User Equipment (UE) 101 can access an Internet Protocol (IP) network 114 via the E-RAN 110.
The ENBs 120-128 correspond to conventional node Bs of the UMTS system, and are connected to the UE 101 via radio channels. Unlike conventional node Bs, the ENBs 120-128 play more complex roles. In the case of the LTE, for example, an apparatus for collecting information regarding the condition of UEs and scheduling them are necessary so that all user traffic, including a real-time service such as Voice over IP (VoIP), is available via a shared channel. To this end, the LTE relies on ENBs 120-128 to schedule the UEs.
In order to implement a maximum transmission rate of 100 Mbps, the LTE is expected to employ Orthogonal Frequency Division Multiplexing (OFDM) as the radio access technology in the 20 MHz bandwidth. In addition, the modulation scheme and the channel coding rate are determined according to the channel condition of UEs, i.e. an Adaptive Modulation & Coding (AMC) scheme will be adopted.
The shared channel plays a role similar to that of a High Speed Packet Data Shared Channel (HS-PDSCH) for High Speed Downlink Packet Access (HSDPA), via which user traffic is transmitted, or that of an Enhanced Uplink Dedicated Packet Channel (E-DPDCH) for an Enhanced Uplink Dedicated Channel (E-DCH).
The LTE also conducts HARQ between the ENBs 120-128 and the UE 101 as in the case of HSDPA or E-DCH. However, the HARQ alone cannot satisfy various requirements on Quality of Service (QoS). Therefore, outer HARQ may be conducted in the upper layer between the UE 101 and the ENBs 120-128.
As used herein, HARQ refers to a technique for increasing the data success ratio of the receiving side by soft-combining previously received data (i.e. packets) with retransmitted packets without discarding the data. Services supporting HSDPA and E-DCH adopt the HARQ scheme so as to increase the transmission efficiency during high-speed packet transmission. The LTE also employs the HARQ scheme between the UE 101 and the ENBs 120-128.
However, adoption of the HARQ scheme inevitably changes the order of packets.
FIG. 2 illustrates a typical HARQ operation and the resulting change of order.
Referring to FIG. 2, the HARQ layer is classified into a transmitting-side HARQ entity 272 and a receiving-side HARQ entity 212 according to the operation. The transmitting-side HARQ entity 272 is adapted to transmit and retransmit HARQ packets, and the receiving-side HARQ entity 212 is adapted for soft-combining of HARQ packets and ACKnowledged/Non-ACKnowledged (ACK/NACK) transmission according to whether or not respective HARQ packets have errors. UEs and node Bs can have both transmitting-side and receiving-side HARQ entities 272 and 212 in response to the downlink or uplink service. Therefore, the following descriptions of the transmitting and receiving sides are not confined to one of UEs or node Bs.
The transmitting and receiving sides have a number of upper layer entities (not shown), a multiplexer 275, and a demultiplexer 210 in order to provide various services through HARQ entities.
The multiplexer 275 is adapted to receive various pieces of data 285 created by various upper layer entities via a transmission buffer 280, insert multiplexing information into the data 285, and transmit the multiplexed data to the receiving-side HARQ entity 272. The demultiplexer 210 is adapted to forward data from the receiving-side HARQ entity 212 to a suitable upper layer entity by using the multiplexing information of the data.
The transmitting/receiving HARQ entities 212 and 272 are the main devices for conducting the HARQ operation, and include a number of HARQ processors 255, 260, 265, 270, 215, 220, 225 and 230. The HARQ processors 255-270 and 215-230 are basic unit devices for transmitting/receiving HARQ packets. The transmitting-side HARQ processors 255-270 are adapted to transmit and retransmit HARQ packets, and the receiving-side HARQ processors 215-230 are adapted to receive and soft-combine HARQ packets and transmit ACK/NACK according to whether or not errors are detected from the HARQ packets.
Sets of transmitting-side and receiving-side HARQ processors 255-270, and 215-230 exist in the transmitting and receiving sides 272 and 212, respectively. Each HARQ entity 272 and 212 has a number of HARQ processors 255-270 and 215-230 so that the HARQ operation is possible without interruption. The HARQ operation includes operations for transmitting HARQ packets by HARQ processors, receiving ACK/NACK in response, and retransmitting the HARQ packets. When a single HARQ processor exists in an HARQ entity, it is not until an HARQ packet is transmitted and corresponding ACK/NACK is received that the next HARQ packet is transmitted. When there are a number of HARQ processors, in contrast, a processor waits to receive ACK/NACK, and another process transmits the next HARQ packet during that time. Therefore, HARQ entities have a number of HARQ processors so that HARQ packets can be transmitted/received without interruption.
The basic operation of HARQ processors will now be described with reference to FIG. 2.
Transmitting side: a transmitting-side HARQ processor, i.e. one of HARQ P1 255, HARQ P2 260, HARQ P3 265, and HARQ P4 270, channel-codes data received from the multiplexing block 275, composes an HARQ packet from the channel-coded data, and transmits the HARQ packet to the receiving side 212. The channel-coded data is stored in a retransmission buffer (not shown) for later retransmission. Upon receiving ACK regarding the HARQ packet from an ACK/NACK transmitter 235, an ACK/NACK receiver 250 flushes the channel-coded data from the retransmission buffer. When NACK regarding the HARQ packet is received from the ACK/NACK transmitter 235, an HARQ packet is composed from the channel-coded data and is retransmitted.
Receiving side: a receiving-side HARQ processor, i.e. one of HARQ P1 215, HARQ P2 220, HARQ P3 225, and HARQ P4 230, which corresponds to the transmitting-side HARQ processor, channel-decodes an HARQ packet received via a physical channel and confirms whether or not the HARQ packet has an error based on a Cyclic Redundancy Check (CRC). If an error exists, data of the HARQ packet is temporarily stored in an HARQ buffer (not shown), and NACK is transmitted to the transmitting side 272 via the ACK/NACK transmitter 235. When retransmission data regarding the HARQ packet is received later, the data stored in the HARQ buffer is soft-combined with the retransmitted data, and another error check is conducted. If it is confirmed that the error still exists, the ACK/NACK transmitter 235 retransmits NACK regarding the HARQ packet again, and repeats the above operation. If it is confirmed that the error has been removed, ACK is transmitted to the ACK/NACK receiver 250, and data in the HARQ buffer is transmitted to the demultiplexer 210.
As mentioned above, the two sets of HARQ processors assigned to the transmitting and receiving sides 272 and 212, respectively, can improve the ratio of successful receipt based on the HARQ operation including retransmission of erroneous HARQ packets and soft combining.
The HARQ operation changes the order of packets. For example, packets are given respective Sequence Numbers (SNs) according to the order of their original creation. Assuming that four HARQ processors are processing packets having an SN of 10, 11, 12, and 13, respectively, i.e. packet 10, packet 11, packet 12, and packet 13, the order in which they are transmitted from the receiving side to the upper layer may differ from that in which they were originally created, depending on how many retransmissions are conducted by respective HARQ processors.
It is assumed that a transmitting-side processor P2 260 has successfully received packet 11 after a single time of transmission, P4 270 has successfully received packet 13 after two times of transmission, P1 255 has successfully received P10 after three times of transmission, and P3 260 has successfully received packet 12 after four times of transmission. Then, the order of packets transmitted from the demultiplexing block 210 to the upper layer is: packet 11, packet 13, packet 10, and packet 12, as indicated by reference numeral 200. When an HARQ processor has successfully received a packet, its error has been removed through a soft combining operation, for example. Particularly, a successfully received packet refers to one confirmed to have no error as a result of the CRC operation.
In order to avoid such a change of order, the transmitting side assigns SNs to transmitted packets according to the transmission order, and the receiving side uses a reordering buffer 205 so that, based on the SNs, the received packets are reordered and transmitted to the upper layer.
However, it is to be noted that an HARQ packet may have a number of multiplexed upper layer packets. This means that, if the upper layer packets are reordered for respective HARQ packets, transmission of an important upper layer packet may be delayed by a packet of little importance. Therefore, the reordering operation is conducted for each flow having similar importance or required QoS of each upper layer packet. For convenience of description, a flow acting as a unit of reordering will be referred to as “reordering flow.” Such a reordering flow may be a flow created by an upper layer entity corresponding to a service, or a flow including a number of service flows having similar importance or required QoS.
FIG. 3 shows a reordering operation for each HARQ packet based on a timer according to the prior art.
Referring to FIG. 3, packets having SNs preceding x−3 have been reordered and transmitted to the upper layer from a reordering buffer 305, and packet x 310 has been transmitted. This means that packet x−2 315 and packet x−1 320 have failed to be received and that they are possibly processed by HARQ processors. At least one packet confirmed to be missing is referred to as a gap 325.
When a gap 325 is detected between received packets, the reordering buffer 305 starts the timer 330. If missing packets corresponding to the gaps 325, i.e. packet x−2 315 and packet x−1 320 fail to be received until the timer 330 stops, it is considered that the missing packets have failed to be received during the HARQ process. Then, packet x 310 is transmitted to the upper layer. In this case, packets 315 and 320 are abandoned from the HARQ layer.
However, the conventional reordering operation based on a timer is very inefficient, because it must take into consideration the worst change of order that may occur during the HARQ process. More particularly, the conventional approach does not consider respective conditions of missing packets, but only the worst condition, and then conducts reordering based on a timer. As a result, reordering requires waiting for an excessively long period of time for the missing packets.